Structure-Activity Relationship of Iron Oxides for NO Reduction in the Presence of C3 H6 , CO, and O2.

Abstract

Spinel-type NiFe2 O4 exhibited the highest NO reduction activity among base-metal oxides under simulated exhaust of a gasoline-powered vehicle. The structure-activity relationship of iron oxides has been investigated through both experimental and computational studies. Spinel iron oxide (γ-Fe2 O3 ) exhibited a much higher NO reduction activity than that of iron oxide with other structures (α-Fe2 O3 and LaFeO3 ). Operando IR measurements clarified that the spinel structure facilitated the reaction between NOx and adsorbed oxidized hydrocarbon or cyanide species. The high reactivity of the spinel structure was ascribed to the high adsorption energy of NO, as elucidated by DFT calculations. Furthermore, molecular orbital calculations demonstrated that the local coordination structure of the spinel iron oxide induced the involvement of not only σ but also π orbitals during NO adsorption on Fe atoms. This work clarified the origin of the structure-dependent activity of metal oxides, with a focus on their local coordination structures.